@Mike Madeja
I would also agree with what others said here, Lombard is a great area, good quality tenants and nice schools. Taxes are really the killer for rental investments in the area though where most properties that are undervalued get purchased by homeowners looking for fixer uppers and outbid most rental investors unless your willing to sacrifice your returns a bit to have a rental in this quality of an area.
@Hae Sun Yi
The whole city of Chicago is hard to compare directly to a town like Naperville as far as rent values, property values and taxes because the city of Chicago is so large and diverse one neighborhood compared to another is really like comparing Naperville to another suburban town.
The main difference that is there between the two though would be tenant laws and rental inspections as they are in two different counties. Cook County (chicago) is much more tenant friendly from what I have heard then Dupage (Naperville) County.